In present power converters, particularly in resonant converters or switching power supplies, diode rectifiers or synchronous rectifiers are used in order to provide for a rectified output power. For high current applications, synchronous rectifiers are the preferred solution, because of their smaller power losses. A synchronous rectifier is a circuit that emulates a diode, allowing current to pass in one direction but not the other without the losses associated with diode rectifiers. In order to minimize power losses, it is very important that the synchronous rectifiers are switched at the correct times to allow current to flow in the correct direction and to avoid a short circuit across the input.
There are two broad techniques to implement the synchronous rectifier function. The first derives a sync signal from the primary controller (which often needs the crossing of a galvanic barrier), whereas the second derives the control signal independently by sensing the electrical conditions. Hence, measuring circuits have to be provided in order to measure the electric conditions like current or voltage.
It is known to use current sense resistors or current transformers in order to measure the current in a certain electrical path. Current sense resistors are cheap and often do not require much space, but cause additional losses. Current transformers are relatively accurate and do not cause additional losses, but often need considerable space.
EP 1 783 788 discloses a power converter having current sensing means. The power converter comprises a magnetic core, a primary winding and two secondary windings which are serially connected. An integrated current transformer with a ring-type core is provided, wherein end portions of the secondary windings of the power converter form primary windings of the integrated current transformer, and wherein a secondary winding is arranged on the integrated current transformer in order to sense the output current in the secondary windings of the power converter. Only one integrated current transformer is needed in order to sense the output current in both secondary windings of the power converter.
A current transformer for measuring a winding current of a power converter needs significant space, even if implemented as an integrated current transformer, and even if currents of several secondary windings are measured with the same device. Therefore, a power converter with an integrated current transformer is relatively large.